The present disclosure relates generally to a system for, and a method of, determining true bearings of radio frequency (RF) identification (RFID) tags associated with items in a controlled area, especially for inventory control of the RFID-tagged items, by using an array of antenna elements for better RF coverage.
Radio frequency (RF) identification (RFID) technology is becoming increasingly important for logistics concerns, material handling and inventory management in retail stores, warehouses, distribution centers, buildings, and like controlled areas. An RFID system typically includes an RFID reader, also known as an RFID interrogator, and preferably a plurality of such readers distributed about the controlled area. Each RFID reader interrogates one or more RFID tags in its coverage range. Each RFID tag is usually attached to, or associated with, an individual item, or to a package for the item, or to a pallet or container for multiple items. Each RFID reader transmits an RF interrogating signal, and each RFID tag, which senses the interrogating RF signal, responds by transmitting a return RF signal. The RFID tag either generates the return RF signal originally, or reflects back a portion of the interrogating RF signal in a process known as backscatter. The return RF signal may further encode data stored internally in the tag. The return signal is demodulated and decoded into data by each reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), or any combination of attributes, and so on.
The RFID tag typically includes an antenna, a power management section, a radio section, and frequently a logic section, a memory, or both. In earlier RFID tags, the power management section included an energy storage device, such as a battery. An RFID tag with an active transmitter is known as an active tag. An RFID tag with a passive transmitter is known as a passive tag and backscatters. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. An RFID tag that backscatters and is powered by an on-board battery is known as a semi-passive tag.
The RFID system is often used in an inventory monitoring application. For example, in order to take inventory of RFID-tagged items in a retail store, it is known to position at least one RFID reader in a controlled area, and then, to allow each reader to automatically read whatever tagged items are in the coverage range of each reader. For superior RF coverage, it is known to provide each reader with an array of antenna elements that transmit the RF interrogating signal as a primary transmit beam that is electronically steered both in azimuth, e.g., over an angle of 360 degrees, and in elevation, e.g., over an angle of about 90 degrees, and that receive the return RF signal as a primary receive beam from the tags.
As advantageous as such known inventory-taking RFID systems utilizing antenna arrays have been, it has proven difficult in practice to very accurately determine the true bearing, i.e., the angular direction both in azimuth and elevation, of a particular tag, relative to a particular reader. There is a practical limit on the number of antenna elements that can be used in each array. This antenna element limit causes each primary transmit beam and each corresponding primary receive beam to have a relatively broad beam width. The primary transmit beam is typically steered until the reader reads the tag with the highest or peak receive signal strength (RSS) of the primary receive beam at a primary steering angle. However, determining the bearing, i.e., the angular direction both in azimuth and elevation, of a tag based on the peak RSS of the primary receive beam is imprecise due to the aforementioned relatively broad beam width. Bearing errors on the order of 5 to 10 degrees have been reported and are not tolerable in many applications.
Accordingly, there is a need to more accurately and finely determine the true bearings of RFID tags despite the practical limit on the number of antenna elements that can be used in an antenna array and despite the relatively broad beam width of the primary transmit and receive beams.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.